1. Field of Invention
The invention relates to a method for forming a silicon oxynitride layer and, in particular, to a method for forming a silicon oxynitride layer, suitable to be used in a fabrication process of semiconductor devices, e.g. poly-silicon (poly-Si) thin film transistors (TFTs).
2. Related Art
Due to its high carrier mobility, low temperature sensitivity, and better driving power, the poly-Si TFT is more suitable for high-speed devices. The liquid crystal display (LCD) that uses the poly-Si as its switching device has a faster and brighter display image. Moreover, the peripheral driving devices and control circuits can be made on the same substrate. The device reliability can be significantly enhanced while the production cost is favorably lowered. Therefore, the poly-Si TFT LCD has become the mainstream development.
In the structure of a conventional poly-Si TFT, a silicon oxide layer is used as the gate insulator. Due to the defects in silicon oxide itself and the defects in the surface bond structure between the silicon oxide and poly-Si, a large leak current may be produced in the poly-Si TFT to result in worse devices. Therefore, it is often necessary to perform annealing at high temperatures. This requires the use of quartz as the substrate. Nevertheless, the cost of quartz substrates is too high. To reduce the cost, one may use glass instead. In that case, the annealing time is longer and its effect is very limited.
Silicon oxynitride has the advantages of both silicon oxide and silicon nitride. It further has a higher thermal stability, low stress, and can effectively reduce the thermal electron effect and the diffusion of alkaline ions (e.g. Na and K ions) in the glass substrate. Therefore, one may first transform the silicon oxide into a nitride as the insulator in the poly-Si TFT. However, the high temperatures required in normal nitridation processes are often above the melting point of the glass substrate. It is then unable to reduce the production cost. Consequently, the rapid thermal annealing (RTA) is developed. In order to utilize this method, additional large equipment is required. Not only does it increase the equipment cost, it is also very hard to integrate the new equipment into the existing low-temperature poly-Si TFT process. At present, the poly-Si TFT has not only a silicon oxide layer but also a silicon oxynitride layer directly formed as the insulator. Relating to the silicon nitride layer, it is almost unused to be as the insulator of the poly-Si TFT. As a result, since a dielectric constant thereof is higher than that of the silicon oxide layer, when the silicon nitride layer is proved as the insulator, a threshold voltage of the device may float and an electron mobility of the device may be lower. Therefore, under the consideration of cost, how to use existing fabrication devices to make better poly-Si TFT's is an important topic in the field of poly-Si LCD.